Inconsistent behaviour when using SpawnRelated with Observers

[Freyja-moth]

Bevy version and features

Main: 71114e1

What you did

Spawning related entities using SpawnRelated is inconsistent with Insert/Add observers.

fn print_name(
    insert: On<Insert, ChildOf>,
    names: Query<&Name>,
) -> Result<(), BevyError> {
    let name = names.get(insert.entity)?;

    dbg!(name);
}

fn spawn_widgets(
    mut commands: Commands
) {

    commands.spawn((
        UiRoot,
        // This will panic
        children!(
            (Button, Name::new("Special Button")), 
        ),
        // This will not panic
        Children::spawn_one((Button, Name::new("Different Button"))),
        // And neither will this
        Children::spawn(SpawnRelated(|parent: &mut RelatedSpawner| {
            parent.spawn((Slider, Name::new("Interesting Slider"));
        })
    ));
}

What went wrong

When using children!() it will panic with The query does not match entity (...).